Sulphidic zinc concentrate often also contains small amounts of other valuable metals, such as copper, lead and silver as well as rarer metals, such as indium, gallium and germanium.
The traditional method for treating sulphidic zinc concentrate is concentrate roasting, in which the sulphidic concentrate is roasted to zinc oxide and the iron in the concentrate forms mainly zinc ferrite. Zinc oxide dissolves fairly easily, so that in the first stage the calcine is subjected to leaching, known as neutral leaching. Zinc ferrite remains undissolved in neutral leaching and strong acid leaching is often used to recover zinc from ferrite. Zinc ferrite precipitate also contains the ferric iron precipitate that was precipitated in neutral leaching. For its part the ferric iron precipitate contains not only ferric hydroxide but also co-precipitated aluminium hydroxide and rare metals such as gallium and indium. Ferrite precipitate may also be fed into a Waelz furnace, in which zinc is evaporated from it, and is then oxidised into zinc oxide and fed back into the leaching process. Waelz oxide can also be processed in a separate process stage in order to recover the other metals such as indium that were co-precipitated into it.
Nowadays the trend has been more and more for processes in which at least some of the sulphidic zinc concentrate is fed directly to leaching without roasting. This enables the processing of impure and fine-ground concentrates. The direct leaching process of zinc sulphide concentrate can be performed in either an atmospheric or pressure leaching process. However, the leaching of zinc sulphide requires a far higher acid concentration than that used in the neutral leaching of calcine, but since the production of elemental zinc almost always occurs electrolytically, the spent acid from electrolysis can be used for concentrate leaching. The highest acid concentration of all is required in the leaching of zinc ferrite formed in roasting. Sulphide concentrate leaching can be combined with a process in which the leaching of ferrites formed in roasting occurs as a strong acid leach and thus ferrite leaching is performed in the same context as concentrate leaching. In this case what is known as a countercurrent leaching process is used, where in addition to a strong acid leaching stage enabling zinc ferrite leaching there is also a low acid leaching stage. A significant part of the concentrate leaching in fact occurs in the low acid leaching (LAL) stage. This type of method is described for instance in U.S. Pat. Nos. 6,475,450, 5,858,315 and 6,340,450 as well as WO publication 2004/076698.
The impurities in the zinc sulphate solution formed in leaching are removed before the solution is routed to electrolysis. Iron is removed by neutralising and oxidising the divalent iron in solution to trivalent, so that the iron is precipitated depending on the conditions as goethite, jarosite or hematite. Other metallic impurities, for example copper, nickel, cobalt and cadmium, are removed mainly by cementing them with metallic zinc powder after iron precipitation. Cementation is based on the oxidation-reduction potentials where the more base metal is oxidised and releases its electrons to the more noble metal in the solution, which is reduced and cemented out of the solution. Zinc is typically recovered from solution by means of electrolysis and impurities, for example cobalt, copper and nickel, diminish the current efficiency of electrolysis. Germanium and cobalt together form a real electrolysis poison, because the metals are precipitated and cause the formation of hydrogen. Indium and gallium do not generally reach as far as electrolysis because they are precipitated out during iron precipitation.
When zinc concentrate contains rare metals such as indium and gallium, it is often desirable to recover them. One possible way to carry out recovery of these metals is to process the neutral leaching residue into Waelz oxide in a Waelz furnace, and then leach the oxide, so that the metals that ended up in the oxide can be put back into the solution and further recovered with solvent extraction. This kind of indium and gallium recovery in the context of a Waelz oxide leaching process is known technology. This process is aided by the fact that these metals have already enriched the Waelz oxide, because they co-precipitate with ferric hydroxide in neutral leaching. In accordance with the method, the zinc oxide containing a valuable metal is leached by means of sulphuric acid, so that in addition to the zinc the indium also dissolves, and the lead and silver and other inert compounds in the oxide remain in the precipitate. The solution is routed to indium extraction, where the indium is separated from the zinc, and the zinc sulphate solution is routed to the neutral leaching stage. If the concentrate contains gallium, its recovery takes place in principle in conjunction with indium recovery, whereupon indium and gallium are separated into their own phases.
U.S. Pat. No. 7,118,719 discloses a zinc process based on calcine leaching, in which iron is subjected to hydrolytic precipitation as jarosite. In the method the solution exiting strong acid leaching is reduced so that the all the iron in solution is reduced to divalent. The reduced solution is routed to neutral leaching. It is stated in the publication that some of the solution entering neutral leaching can be routed to a sidestream and neutralised to a pH value of at least 4, so that an iron-free precipitate containing Ga, In and Ge is obtained.
A method is disclosed in patent publication FI 118226 for recovering at least one rare metal, such as indium and/or gallium in conjunction with zinc sulphide concentrate leaching. Zinc sulphate solution, which is generated during direct concentrate leaching and contains iron and rare metals, is routed to a neutralisation and precipitation stage, in which the solution is neutralised to a pH range of 2.5-3.5. The amount of trivalent iron in the solution is controlled to be such that it is sufficient to co-precipitate the indium and/or gallium in the solution.